1. Field of the Invention
The present invention relates to a light emitting device in which a plurality of pixels each comprises a light emitting element and a means for supplying current to the light emitting element.
2. Description of the Related Art
As a driving method of a light emitting device, there is a time gray scale method in which a gray scale is displayed by controlling a light emitting period of a pixel in one frame period by use of binary voltage of a digital video signal. Electroluminescent materials are more suitable for a time gray scale method than liquid crystals and the like since the response speed is generally faster. Specifically, when displaying by the time gray scale method, one frame period is divided into a plurality of subframe periods. Then, a pixel emits light or does not emit light according to a video signal in each of the subframe periods. According to the aforementioned structure, the total actual light emitting period of a pixel in one frame period can be controlled with a video signal, so that a gray scale can be displayed.
However, in the case of performing display using the time gray scale method, there is a problem in that a pseudo contour may be displayed in a pixel portion depending on the frame frequency. Pseudo counters are unnatural contour lines that are often perceived when a middle gray scale is displayed by the time gray scale method, which is considered to be caused by a variation of the perceptual luminance due to a characteristic of the human sight.
A pseudo contour includes a moving image pseudo contour which occurs when a moving image is displayed, and a still image pseudo contour which occurs when a still image is displayed. The moving image pseudo contour occurs because in contiguous frame periods, a subframe period included in the previous frame period and a subframe period included in the present frame period are perceived as one continuous frame period by human eyes. That is, moving image pseudo contours mean unnatural bright or dark lines displayed in a pixel portion that are perceived by human eyes since the number of gray scales deviates from the number of gray scales to be displayed in the actual frame period. A mechanism for generation of a still image pseudo contour is the same as that of a moving image pseudo contour. The still image pseudo contour occurs when a still image is displayed, because a human view point slightly moves horizontally or vertically at a boundary between regions having the different numbers of gray scales, and thus a moving image seems to be displayed at pixels in the vicinity of the boundary. That is, still image pseudo contours mean unnatural bright or dark lines that occur in a swinging manner in the vicinity of a boundary between regions having the different numbers of gray scales due to a moving image pseudo contour that occurs at pixels in the vicinity of the boundary.
In order to prevent the above-described pseudo contours, it is effective to increase the frame frequency, or to further divide the subframe period into a plurality of frames. Patent Document 1 has been disclosed a technology for preventing light emitting periods of a pixel or of non-light emitting periods of a pixel from occurring continuously by dividing a subframe period into a plurality of frames.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-149113
When a subframe period is divided into a plurality of frames, the larger the dividing number is, the more certainly generation of a pseudo contour can be suppressed. However, the larger the dividing number is, the shorter the divided subframe period is required to be. Similarly, in the case where the frame frequency is increased, each subframe period is required to be shortened as well.
Furthermore, in the case of an active matrix type light emitting device, it is necessary to input a video signal into pixels at all rows for each subframe period or divided subframe period. Therefore, if subframe periods or divided subframe periods are shortened, the next subframe period or divided subframe period starts before video signal input for all rows in a pixel portion is completed. However, in active matrix type light emitting devices generally, each pixel comprises a light emitting element, a transistor for controlling video signal input into the pixel (switching transistor), and a transistor for controlling a current value supplied to the light emitting element (driving transistor). Consequently, it is impossible to input video signals into the pixels at two or more rows in the pixel portion in parallel.
Therefore, the drive frequency of a driver circuit is required to be increased such that video signal input at all rows in a pixel portion is completed before the next subframe period or divided subframe period starts even if the subframe period or divided subframe period is shortened. From the viewpoint of reliability of a driver circuit, however, it is not preferable to increase the drive frequency more than is necessary.